Method and apparatus for controlling output current of turbine/alternator on common shaft

ABSTRACT

An electrical system and method for a turbine/alternator comprising a gas driven turbine and a permanent magnet alternator rotating on a common shaft includes an inverter circuit connectable either to an output circuit or the stator winding of the alternator. A control circuit during a start-up mode switches the inverter circuit to the stator winding of the alternator and during a power out mode switches the inverter circuit to the output circuit. During the power out mode, current data is sensed and is used for current limit and/or power balancing.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a division of U.S. patent application Ser.No. 10/466,386 filed May 28, 2003 which is a continuation of U.S. patentapplication Ser. No. 09/840,572, filed Apr. 23, 2001, which is acontinuation of U.S. application Ser. No. 09/319,390, filed Jun. 1,1999, which is the United States national phase of InternationalApplication No. PCT/US97/22405, filed Dec. 3, 1997 which designated,inter alia, the United States, and which claims the benefit of U.S.Provisional Application No. 60/032,149, filed Dec. 3, 1996.

BACKGROUND OF THE INVENTION

[0002] Gas turbines must be driven to rotate at a starting speed byauxiliary means prior to fuel injection and ignition and self-sustainedoperation. In the past, for example, gear box systems driven byauxiliary electric or compressed air motors have been used to rotate theturbine to starting speed. “Air” impingement starting systems have alsobeen used with small turbines and operated by directing a stream of gas,typically air, onto the turbine or compressor wheel to cause rotation ofthe main rotor. These prior art systems are complex and difficult toimplement.

[0003] Electrical power may be generated by using a gas turbine to drivean alternator. The alternator may be driven by a free turbine which iscoupled to the rotor of the alternator or through a gear box. In thesesystems, the speed of the turbine must be precisely controlled tomaintain the desired frequency and voltage of the alternating currentoutput.

SUMMARY OF THE INVENTION

[0004] In accordance with the present invention, an alternator having apermanent magnet rotor is connected to the main turbine rotor makingpossible both starting of the turbine as well as generation ofelectrical power. The electrical system described herein allows therotor to operate at various speeds with an output frequency and voltageunrelated to rotor speed. The electrical system incorporates a uniqueinverter which yields the appropriate voltage and frequency in both thestartup mode of operation as well as in the power generation mode ofoperation.

[0005] The electrical system is used to cause rotation of the turbineduring the startup mode and subsequently is used to extract electricalpower from the alternator after the turbine has reached its normaloperating conditions. At startup, the alternator functions as anelectric motor. The functions of the electrical system at startupcomprise power boost, power switching and control to provide, forexample, three-phase AC electrical power to the alternator. Both thefrequency and voltage are controlled as a function of time and rotationspeed. Electrical power for the electrical system is obtained duringstartup from either a DC source, such as a battery, or from an AC powerline. The startup circuit may function as an open loop control system oras a closed loop control system based upon rotor position feedback.

[0006] As the turbine approaches normal operating conditions at veryhigh speeds of rotation powered through the controlled combustion offuel and air, the electronic circuitry used to initially drive thealternator as a motor is automatically reconfigured to accept power fromthe alternator. Subsequently, three-phase electrical power becomesavailable for extraction from the electrical system at desired voltagesand frequencies.

[0007] Briefly, according to this invention, an electrical system for aturbine/alternator comprises a gas driven turbine and alternatorrotating on a common shaft. The alternator has a permanent magnet rotorand a stator winding. A stator circuit is connected to the statorwinding. A DC bus powers an inverter circuit. The output of the invertercircuit is connected to an AC output circuit or through a firstcontactor to the stator circuit. A rectifier is connected between thestator circuit and the DC bus. A signal generator is driven by signalsderived from the rotation of the common shaft and an open loop waveformgenerator produces waveforms independent of the rotation of the commonshaft. A second contactor connects either the signal generator or theopen loop waveform generator to a driver connected to cause switching ofthe inverter circuit. A temporary power supply supplies energy to the DCbus. A control circuit, during a startup mode, switches the firstcontactor to connect the inverter circuit to the stator circuit andswitches the second contactor to connect the signal generator to thedriver, preferably a pulse width modulator. The control circuit, duringa power out mode, switches the first contactor to disconnect theinverter from the stator circuit and switches the second contactor toconnect the open loop waveform generator to the driver. During thestartup mode, the alternator functions as a motor to raise the speed ofthe turbine to a safe ignition speed. The inverter is used to commutatethe stator windings in response to the signal from the signal generator.During a power out mode, the inverter is used to convert the rectifiedoutput of the alternator into AC signals applied to the AC outputcircuit in response to the open loop waveform generator, thus producingelectric power having a frequency unconnected to the rotational speed ofthe alternator.

[0008] According to a preferred embodiment, an electrical system for aturbine/alternator comprises a gas driven turbine and alternatorrotating on a common shaft. The alternator is comprised of a permanentmagnet rotor and a stator winding. The stator winding is connectedthrough a contactor to an inverter circuit. The inverter circuit isconnected to a DC bus. The inverter circuit is also connected to asignal generator. A position encoder is connected to the drive shaft ofthe turbine/alternator. Its output is also connected to the signalgenerator. The inverter processes the DC bus voltage and signalgenerator output to develop three-phase AC output voltages. The signalgenerator controls the inverter output frequency. Concurrently, avariable voltage DC power supply applies a time variant voltage to theDC bus. The DC bus voltage controls the inverter output voltage level.Thus, the output frequencies and voltages of the inverter are fullycontrollable. During the startup mode, the output of the inverter isapplied through a contactor to the alternator which functions as anelectric motor. When the startup mode is initiated, the DC power supplyvoltage begins to ramp up from 0 volts. The signal generator outputfrequency is set to a fixed low frequency. As the DC bus voltage beginsto increase, the alternator rotor begins to rotate at a low speed. Theencoder senses shaft position changes and sends this information to thesignal generator. The signal generator processes this information andbegins to ramp up its output frequency as a function of engine speed.This increasing frequency is directed to the inverter where it is usedto control the frequency of the inverter output voltage. This controlledprocess results in a time variant inverter output whose frequency andvoltage are applied through a contactor to the alternator. As a result,the alternator functions as a motor and accelerates the speed of theturbine shaft to a value suitable for ignition. Once the turbine hasreached its normal operating speed, the variable voltage power supply isdeactivated. Further, the shaft position encoder signal is disconnectedfrom the signal generator and is replaced by a precision, fixed timebase signal. Subsequently, the alternator AC output voltage is rectifiedand the resulting DC output voltages are applied to the DC bus. Thisreconfiguration permits the inverter to operate as a fixed frequencypower output source independent of turbine rotor speed. In the poweroutput mode, the inverter provides power through output filters. Thefiltered output power is then connected to a contactor which directs itto a set of terminals where it is available for consumer use. A controlsystem integrates operation of the inverter, power supply, signalgenerator and contactors during both the startup and power output modesof operation. During the power output mode of operation, the controlsystem continuously measures output voltages from the inverter and sendssignals to the signal generator to compensate for output voltagefluctuations caused by varying output load conditions.

[0009] According to a preferred embodiment, the signal generator is apulse width modulator. Typically, the stator winding of the alternatoris a three-phase winding and the inverter circuit and the AC circuitsare three-phase circuits.

[0010] According to a preferred embodiment, the electrical systemcomprises a battery powered supply circuit including a battery and aboost from 0 inverter circuit for outputting to the DC bus a voltagebetween 0 and that required by the inverter to power the alternator tosafe ignition speeds. According to another preferred circuit, thebattery powered supply circuit comprises a step-down circuit forrecharging the battery and for powering low voltage devices such as fansand pumps from the DC bus during the output mode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Further features and other objects and advantages will becomeclear from the following detailed description made with reference to thedrawings in which:

[0012]FIG. 1 is a schematic drawing showing the overall relationship ofthe electrical system to the gas turbine/alternator;

[0013]FIG. 2 is a schematic drawing showing the electrical system forproviding electrical power to the alternator during the startup mode andfor passing power generated to the load during the power out mode;

[0014]FIG. 3 schematically illustrates a rectifier circuit forconverting the alternator output to a DC current voltage on the DC bus;

[0015]FIGS. 4a and 4 b schematically illustrate the inverter circuitcomprised of six IGBT switches used to commutate the current to thealternator during the startup mode and to provide three-phase outputduring the power out mode;

[0016]FIG. 5 schematically illustrates the open loop waveform generatorand closed loop driver for the inverter circuit;

[0017]FIG. 6 illustrates a boost/buck chopper suitable for using batterypower during the startup mode to power the DC bus and for charging thebattery from the DC bus during the power out mode; and

[0018]FIG. 7 schematically illustrates the entire electrical systemincluding turbine sensors and turbine controls.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019]FIG. 1 illustrates the relation between the electrical controlsystem 1, according to this invention, and the power generation systemcomprising a gas turbine 2 and an alternator 3. The alternator armatureis mounted on a shaft common with the turbine shaft. The electricalcontrol system interacts with the power generation system to providestartup power, engine control, signal processing, battery charging, userinterfaces, as well as power conversion and control for generating userpower. Both stand-alone and line tie operations are facilitated.

[0020] Referring now to FIG. 2, the general arrangement of theelectrical power circuits for a turbine generator, according to thisinvention, is depicted. A turbine 10 is connected to a permanent magnet(rare earth samarium-cobalt) alternator 11 by a common shaft 12. Thestator is manufactured using a stack of high quality, low loss, electricsheet steel laminations. This stack contains a three-phase distributedwinding in twelve stator slots with a housing with provision for oilcooling. The performance of the alternator depends upon effectivecooling. In the currently implemented embodiment, the four polepermanent magnet rotor has the following dimensions: active length 3.55inches; diameter under magnets 1.00 inch; diameter over 1.430 inches;weight of magnets 0.70 pounds; rotor weight 1.95 pounds.

[0021] The three-phase stator windings of the alternator are connectedby an AC bus 14 to a rectifier 15. The output of the rectifier isconnected to a DC bus 16. During power generation, that is, the powerout mode when the turbine is driving the alternator, the three-phaseoutput on the AC bus is rectified by the rectifier providing DC power onthe DC bus. The DC power is applied to an inverter 17. The inverter 17during the power out mode switches the DC power to provide three-phaseoutput having a frequency unrelated to the rotational speed of thealternator. The frequency is controlled by signals from a systemcontroller 18. The inverter output is filtered by inductors 19 andcapacitors 20. The filtered three-phase output is passed to a loadthrough an output contactor 21 (controlled by the system controller 18through a relay 22) and output breakers 23.

[0022] A current transformer 25 senses output current which is fed backto the system controller 18 enabling current limit and power balancingof the three-phase output.

[0023] In order to start the turbine, it is necessary to accelerate itto a suitable ignition speed. During the startup mode, the alternator isoperated as a motor. During the startup mode, the output of the inverter17 is connected to the stator windings of the alternator 11 through astart contactor 30 which is controlled by the system controller 18. Atthe same time, a capacitor contactor 31 removes the filter capacitors 20from the output circuit. Because of the very high frequencies duringstartup, it is necessary to remove the filter capacitors 20 from thestator circuits.

[0024] During startup, DC power is drawn from a battery 33 through afuse 34 and is applied to a boost chopper 36. The boost chopper rampsthe voltage of the DC battery power from 0 to a voltage which, whenconverted to AC by the inverter 17, will drive the alternator as a motorat a speed that will enable safe ignition of the turbine. Preferably, ashaft position sensor 37 generates a signal which is applied to thesystem controller 18 which in turn uses the signal to control theinverter 17 to generate a three-phase output which commutates the statorwindings of the alternator to ramp the alternator and turbine up toignition speed.

[0025] Referring to FIG. 3, a suitable rectifier circuit isschematically illustrated. The three-phase stator windings 40, 41, 42,delta connected, are connected as illustrated by six diodes 43 a, 43 b,43 c, 43 d, 43 e, 43 f to the DC bus 16.

[0026] Referring to FIGS. 4a and 4 b, a suitable inverter circuit isschematically illustrated. (FIG. 4a illustrates a delta connection andFIG. 4b a star connection for the stator winding.) The invertercomprises six solid state (IGBI) switches which, during the startupmode, can alternately connect one corner of the delta connected statorwindings to the plus or minus side of the DC bus 16 through contactor30. Also, the solid state switches 44 a, 44 b, 44 c, 44 d, 44 e, 44 fconnect either the plus or minus side of the DC bus to the filterinductors 19 at all times and after startup to the filter capacitors 20through contactor 31. The inverter is used to generate three-phaseoutput signals. It is capable of providing a wide variety of outputvoltages and frequencies as controlled by a microprocessor in the systemcontroller. The output inverter is used in two distinctly different waysduring startup and power out operations of the power generation system.

[0027] During the startup phase, the inverter is used to output timevariant voltages and frequencies needed to drive the alternator as amotor and to accelerate the alternator turbine drive shaft to rotationspeeds necessary for sustained operation of the power generation system.In its present configuration, this requires three-phase voltages rangingfrom 0 up to 350 volts at frequencies from near 0 and up to 2 kHz.

[0028] During the power out phase, the inverter is used to outputthree-phase voltages consistent with user power requirements. Typicalvoltages are 480 vac, 240 vac, 208 vac, 120 vac at frequencies of 50, 60and 400 Hz. This system is not limited to these values and a nearlyinfinite range of voltages and frequencies could be selected if desired.

[0029] Certain applications of the power generation system require theoutput inverter to be capable of line tie to an existing power grid.Line phasing circuitry is used in conjunction with a system controllerto monitor the phase of the power grid voltage and synchronize the powergeneration system to it. In like manner, the system controller canmonitor power grid voltage amplitudes and adjust the power generationsystem output to facilitate and control the transfer of power to thegrid.

[0030]FIG. 5 schematically illustrates the portion of the systemcontroller for generating an open loop waveform for driving the inverter17. A frequency generator 50 generates output pulses at frequenciesselectable between 250 Hz and 600 kHz by a CPU 51. These pulses areapplied to advance the output in sine wave PROMs (programmable read onlymemories) 52 a, 52 b, 52 c. The outputs from the sine wave PROMs(basically a 256K lookup table) are phase shifted from each otherexactly 120° apart. The outputs from the PROMs are applied todigital-to-analog converters 53 a, 53 b, 53 c, producing three analogsine waves. The amplitude of each waveform out of the digital-to-analogconverters is individually controlled by a sine wave (amplitude)command. The sine waves are then compared in pulse width modulators 54a, 54 b, 54 c with a triangle wave from a triangle wave generator. Thefrequency of the triangle wave generator is controllable. The pulsewidth modulated waveforms are then applied through drive select gates 55a, 55 b, 55 c to drivers 57 a, 57 b, 57 c. In the currently implementedembodiment, the drivers produce three complimentary pairs of pulsesignals for controlling the inverter. The waveform generator is used todrive the inverter during the power out mode when the turbine is drivingthe alternator. The waveform circuit, so far as described, is open loop.In other words, it is not controlled by alternator rotation speed.However, various feedback signals can be used to adjust the amplitude ofsignals out of the digital-to-analog converter. While the waveformcircuit is principally used to drive the inverter during the power outmode, it may be used to control the inverter at the very beginning ofthe startup mode to cause the armature to rotate at least once. Thispermits phasing of the Hall effect sensor signals.

[0031] Three Hall effect switches 58 are mounted to pick up magneticpulses 120° apart as the common shaft rotates. These signals areprocessed by a Hall logic circuit 59 to produce a pair of signalscorresponding to each pickup pulse. The three pairs of signals are gatedby the drive select gates 55 a, 55 b, 55 c to the drivers 57 a, 57 b, 57c. The position sensor system consists of permanent magnets and Halleffect sensors which are used during turbine engine startup to commutateelectrical power to the stator windings of the alternator. Phasing ofthe sensors is accomplished at the beginning of the startup phase bybriefly rotating the turbine alternator shaft in the direction of normalrotation. Rotation of the shaft during this initial period of thestartup phase is accomplished by the microcomputer control of the outputinverter system in an open loop configuration that does not utilize theHall effect sensors. Once phasing of the sensors has been completed,their signals are directed to the output inverter section of the systemto facilitate startup of the turbine engine under closed loop control.The Hall effect pickups enable a closed loop commutation of the inverter17 and the stator windings of the alternator. A gain control circuit 61processes feedback from the inverter circuit 17 to adjust the gain ofthe driver circuits to balance the output of the three phases outputfrom the inverter 17.

[0032] During the startup mode, the battery supplies power to the DC busthrough the boost chopper. FIG. 6 is a schematic of a boost chopper forsupplying the DC bus with a voltage of 0 to 350 volts from a 12 or 24volt battery during the startup mode. When the boost chopper switches 65a and 65 b are closed (conducting), current flows in an inductor 66.When the switches 65 a and 65 b are open, the magnetic field in theinductor collapses driving end A of the inducter very positive withrespect to end B and supplying current through diodes 67 a and 67 b tothe positive and negative sides of the DC bus, respectively. Theswitches 65 a and 65 b are driven at 4 kHz. The duty cycle is controlledfrom 0 to 100% enabling the output voltage across DC bus capacitors 70to vary from 0 to 350 volts. The use of a boost from 0 chopper circuitenables a gradual increase in the rotational speed of the alternatorduring startup.

[0033] During the power out mode, the battery is charged by a chargercircuit. Charger switches 68 a and 68 b are switched at about 1 kHz. Theduty cycle is adjustable. When the charger switches 68 a and 68 b areclosed, current from the DC bus flows through inductor 66. When thecharger switches are opened, side B of the inductor goes positive withrespect to side A and charges the battery drawing current through diodes69 a and 69 b. It is not necessary, as illustrated here, that the boostand charger circuits share the same inductor.

[0034] In the preferred embodiment of this invention designed for a 45kW power output, the following components are sized as set forth: filterinductors 19  300 mH per phase filter capacitors 20  100 μF per phase DCbus capacitor 70 4700 μF IGBT switches in inverter 17  400 A/600 V

[0035]FIG. 7 illustrates the interaction between the system controllerand the gas turbine. The system controller utilizes threemicroprocessors that communicate with each other through a high speedserial link and provide the following functions: (1) control of theelectrical power required to rotate the turbine rotor up to speedsnecessary to sustain operation of the turbine; (2) process and controlof the electrical power generated by the alternator during power outsystem operation to provide three-phase output power at common linevoltages and frequencies; (3) control of other subsystems needed tooperate the power generation system, such as the ignitor, cooling fans,fuel and oil pump; (4) signal conditioning and control ofinstrumentation for measurement of pressures, temperatures, flow andspeed; and (5) generation and control of a control panel providing auser interface for system operation and diagnostics.

[0036] The three microprocessors each have their own associated memoryprogrammed to run independently. One microprocessor is directed tomonitoring the keypad, display and RS232 communicators. A secondmicroprocessor is devoted to monitoring the turbine parameters, toactuate fault trips and to log a history of operation parameters for thelatest hour of operation. The third microprocessor monitors and directsthe electrical circuit selected frequencies, voltages, actuates relays,etc.

Operation

[0037] There are two separate modes of system operation. In the firstmode, the system controller 18 is used to control the boost chopper 36and output inverters 17 to vary the output voltage and frequency as afunction of time. Operating in this manner, the alternator is utilizedas a variable speed motor to rotate the engine at speeds required forthe gas turbine sustained operation. In the second mode of operation,the inverter section is automatically reconfigured by the systemcontroller 18 for providing user power output. In this mode ofoperation, high frequency AC power output from the alternator isconverted to DC power by the rectifier 15 and applied to the input ofthe inverter. The inverter, in conjunction with the system controller,provides the desired three-phase output voltages and frequenciesrequired in normal user applications. The output voltage frequency andphase are controlled in a manner consistent with stand-alone and linetie user applications.

[0038] The control panel 72 provides the interface between the user andthe controller. It provides the user with various control andinstrumentation options, such as startup, shut down, line tie anddiagnostics. During normal startup and operation of the system, thesystem controller sequences and controls the power generation system asfollows.

[0039] 1) On command from the control panel 72, the controller 18 sendsappropriate commands to the waveform generators and boost chopper toinitiate brief rotation of the turbines so that the Hall positionsensors are properly phased for subsequent startup functions.

[0040] 2) Next, the controller controls the boost chopper 36 and thewaveform generator (see items 50 to 54 and 58 in FIG. 5) to ramp upthree-phase voltages and frequencies to the inverter. The three-phaseoutputs are directed to the alternator which responds by acceleratingthe rotation of the turbine shaft to speeds necessary for its sustainedoperation.

[0041] 3) During the above startup sequence, the system controllermonitors and controls other functions, such as fuel flow, ignition,rotation speeds, temperatures and pressures.

[0042] 4) Following the startup phase, the system controllerreconfigures the boost chopper to operate as a battery charger. Inaddition, the waveform generator is reset to provide signals needed forgeneration of user power output requirements. These signals areconnected to the input of the selector switch where they are directed tothe drivers and inverter. As a result, the inverter provides the desiredthree-phase output voltages and frequencies desired by the user.

[0043] 5) During normal power out operation as described in 4) above,the system controller monitors and controls all functions necessary forcontrol of the power generation system including, but not limited to,control and/or monitoring of fuel flow, temperature, pressure, speed,run time and various diagnostics unique, to the components of thecomplete power generation system.

[0044] Having thus described our invention with the detail andparticularity required by the Patent Laws, what is desired protected byLetters Patent is set forth in the following claims.

What is claimed:
 1. A method of controlling a turbine/alternatorcomprising a gas driven turbine and permanent magnet alternator on acommon shaft comprising: providing electric power to saidturbine/alternator through an inverter circuit to start saidturbine/alternator to achieve self-sustained operation of saidturbine/alternator; and reconfiguring said inverter circuit to outputelectric power from said turbine/alternator when self-sustainedoperation of said turbine/alternator is achieved, wherein current dataof the electric power is sensed by a sensor during outputting electricpower from said turbine/alternator.
 2. The method of claim 1, whereinsensed current data of said sensor is used to provide current limitcontrol.
 3. The method of claim 1, wherein sensed current data of saidsensor is used to provide power balancing control.
 4. The method ofclaim 3, wherein said inverter circuit comprises an inverter, and saidsensed data of said sensor is used to provide power balancing control ofa three-phase output of said inverter.
 5. The method of claim 1, whereinsaid sensor comprises a current transformer.
 6. The method of claim 1,wherein during providing electric power to said turbine/alternator,controlled combustion of fuel and air is provided to said gas driventurbine of said turbine/alternator.
 7. The method of claim 1, whereinwhen reconfiguring said inverter circuit, said inverter circuit isconnected to said turbine/alternator through a rectifier.
 8. The methodof claim 1, wherein said inverter circuit comprises an output filter forfiltering said electric power and said output filter is removed whenproviding electric power to said turbine/alternator through saidinverter circuit.
 9. An electric system for a turbine/alternatorcomprising a gas driven turbine and permanent magnet alternator on acommon shaft comprising: an inverter provided for operation of saidturbine/alternator; means to provide electric power to saidturbine/alternator through said inverter to start saidturbine/alternator to achieve self-sustained operation of saidturbine/alternator; means to reconfigure said inverter to outputelectric power from said permanent magnet turbine/alternator to supplythe electric power to a load; and a sensor for sensing output current ofsaid turbine/alternator.
 10. The electric system of claim 9, whereinsensed current data of said sensor is used to provide current limitcontrol.
 11. The electric system of claim 9, wherein sensed current dataof said sensor is used to provide power balancing control.
 12. Theelectric system of claim 11, wherein said sensed current data of saidsensor is used to provide power balancing control of a three-phaseoutput of the inverter.
 13. The electric system of claim 9, wherein saidsensor comprises a current transformer.
 14. The electric system of claim9, further comprising means to provide controlled combustion of fuel andair to said gas driven turbine to achieve self-sustained operation ofsaid gas driven turbine.
 15. The electric system of claim 9, whereinsaid means to reconfigure said inverter connects said inverter to saidturbine/alternator through a rectifier.
 16. The electric system of claim9, wherein said load comprises a power line, said means to reconfiguresaid inverter to supply electric power to said power line at a commonline voltage and frequency.
 17. The electric system of claim 9, furthercomprising an output filter for filtering said electric power, whichoutput filter is removable when said means to provide electric powerprovides electric power to said turbine/alternator.